Slowtwitch Classifieds Forum
eBay & Craigslist:
Bicycle Blue Book:
The Pro's Closet: If you don't want the hassle of selling it yourself, not getting paid, having the buyer return it, or conversely the risks associated with being the buyer; and/or you want an expert set of eyes on a used product before you buy it; this is the benefit of The Pro's Closet. This company is now also an institutional option for certain brands, as in, you go to your LBS, trade in your bike or wheels pursuant to a new product. We'll update you here on the list of companies that participate in this.
Trek and BBB: Trek has a system involving Bicycle Blue Book and its local retailers facilitating trade-ins pursuant to a new Trek purchase. More details on this as we find out.
Your Local Bike Shop: Some shops, PlayTri one example, have been taking trade-ins, and trafficking in the used market, for years. We'll endeavor to maintain a list here.